1. Field of the Invention
The invention relates to a decision feedback device for the filtering of a digital signal, working in the frequency domain. The invention can be applied, for example to the cancellation of echoes generated by multipath propagation during the RF transmission of a digital signal in particular.
2. Description of the Related Art
A major problem in terrestrial transmission systems is that of signal distortion due to the phenomenon of multipath propagation. The reflection of a transmitted signal on dwellings, relief features or various layers of the atmosphere gives rise to the reception of several signals or echoes instead of only one signal. The undesirable echoes received are generally out of phase and time-lagged, and have amplitudes that are attenuated as compared with the origin signal (all in a manner that is variable in time, depending on the path followed). Since these phenomena depend, as far as their variation is concerned, chiefly on weather phenomena, they change slowly for a fixed reception system.
In digital transmission, the echoes received induce a distortion of the received symbols. This distortion results in a phenomenon of overlapping of the symbols, also called intersymbol interference. In order to ensure high quality reception, the resulting intersymbol interference must be eliminated or substantially reduced.
The pulse response of a multipath channel extends over a typical time interval of some tens of microseconds. This interval, which is a function of the frequency used, corresponds typically to a few hundreds of successive symbols in digital television broadcasting systems. One of the symbols received (see FIG. 2) corresponds to a symbol (x(n)) that is actually transmitted at the beginning, and the other symbols (x(n+i), x(n-j)) are parasitic symbols (or echoes) resulting from the multipath propagation. One processing approach involves the use of high-order digital filters, i.e. a large number of corrective coefficients, at reception, in order to eliminate the parasitic symbols (and produce, through a decision device, a sequence of decisions actually corresponding to the transmitted data).
The sampled pulse response of a multipath channel can be put in the following form: EQU h(t)=.delta..sub.k (t)+.SIGMA..sub.k .alpha..sub.k .SIGMA.(t-t.sub.k),
with K as an index that is a integer ranging from 1 to p in considering p paths, .alpha..sub.k and t.sub.k being the effective gain and temporal delay respectively of the kth path, (t) being transmitted pulse. The gains are generally complex. The time delays t.sub.k are positive (echoes known as postcursor echoes) or negative (echoes known as precursor echoes). In practice the precursor echoes are temporally very close to the signal relating to the main path (typically at a temporal distance of less than 1 microsecond) whereas the postcursor echoes are fairly spread out temporally (being typically received with a time lag of 0 to 40 microseconds).
As we have seen, the phenomenon of multipath propagation changes over time, and it is therefore necessary to adapt the filtering coefficients temporally. Filters known as adaptive filters are used to this end. These adaptive filters, according to their principle, comprise a variable coefficient filter and a computation device providing these coefficients to the filter as a function, firstly, of the sequences of symbols received and produced by the filter, and, secondly, of the sequences of corresponding decisions. The coefficient adaptation devices are physically uncoupled for high sampling rates (for example rates of over one megahertz). The adapting of the coefficients is usually done in a digital signal processor or DSP, and the filter is normally made on a VLSI (Very Large Scale Integration) circuit. An approach of this type is described, for example, in the document EP-A-0 641 102. The temporal type processing is conventionally carried out therein symbol by symbol.
In terms of adaptation of coefficients, a known approach lies in the use of adaptation techniques known as techniques of adaptation on the basis of blocks of symbols: the coefficients are prepared out of blocks of symbols. Thus, a faster convergence is obtained than in the case of step-by-step algorithms. Fast linear filters are then used to carry out time domain filtering, symbol by symbol, or frequency domain filtering by blocks of symbols. A linear filtering of this kind, performed in the frequency domain, makes it possible, as compared time domain filtering, to reduce the number of computations through the replacement of a temporal convolution by a frequency multiplication. An approach of this kind, for example, is described in the article "On The Convergence Properties Of Partitioned Block Frequency Domain Adaptive Filter (PBFDAF)" in Torres L., Masgrau E., Lagunas M. A. (eds), Signal Processing V: Theories and Applications; Proceedings of EUSIPCO--90, Fifth European Signal Processing Conference, Barcelona, Sep. 18-21, 1990, Vol.1, Sep. 18, 1990, pages 201-204.
Nevertheless this filtering structure does not make it possible, with an accessible degree of complexity, to obtain sufficient correction in the case of the long echoes, in particular when the temporal spread of the echoes exceeds about a hundred symbols.